I'm pretty ambivalent toward Katy; she's clearly capable of making great pop, as evidenced by such masterpieces as "Teenaged Dream" and "Birthday," but most of the rest of her catalog is middling. She's pretty spotty as a songwriter, both lyrically and melodically, and her persona is fairly uninteresting.
When she gets me in that sweet spot with one of her songs I'm hooked, but when she doesn't I don't have much use for her.